Freeze-drying is used for the preservation of a wide variety of foods, pharmaceuticals, and biological products. Extreme care must be taken in handling and processing many of these products to minimize opportunities for contamination. For example, freeze-drying equipment is often steam-sterilized between batches, and in many cases the entire operating area in which the equipment is located may be outfitted as a sterile clean room to minimize the exposure of products to contaminants as they are being transported to and from the freeze-dryer. In many cases, products must be re-packaged after freeze-drying, thus presenting yet another handling step that provides an opportunity to introduce contaminants into the freeze dried product.
Many freeze-drying processes involve placing open containers of material in the freeze-dryer. Containers are kept open until the freeze-drying process is completed to allow a path for water vapor to be removed from the product. This practice, however, presents an opportunity for contamination; hence the concern for cleanliness and sterility of the freeze-drying equipment and the area surrounding it.
Cross-contamination between different batches of product being dried at the same time is also a problem. Freeze-drying equipment is expensive, and freeze-drying cycles are generally very long, consuming many hours or even several days for the processing of a single batch of material. As a result, it is very common for freeze-dryers to maximize the use of their capital investment in the equipment by attempting to fully load the freeze-drying chamber every time it is cycled. This in turn results in the common practice of freeze-drying different materials in the same chamber at the same time. Since all the materials are in open containers, cross-contamination of product can, and commonly does, occur.
For example, in U.S. Pat. No. 3,454,178 to Bender, et al., a vial contains a slotted vial cap that, when in the "up" position, allows a path for water vapor to escape the vial. Vials are introduced into the process with their caps in the "up" position, and remain that way until the drying cycle is complete. At the end of the cycle, freeze-drier shelves squeeze down on the vials and press the caps into the "down" position, thus sealing the vials before the drier door is opened. This approach assures that contents of the vials are not contaminated after the process is complete. It also assures that water vapor cannot enter the vials and rehydrate the product once the drier doors are open; indeed, the vials are often repressurized at the end of the process with a dry inert gas, such as nitrogen, prior to pushing the vial caps into the "down" position, to maximize the shelf life of the freeze-dried product. But the problem of contamination of the vial contents when the vials are being loaded into the drier or during the freeze-dry process itself is not addressed by this patent.
In European Patent No. 343,596, a container that has been designed to protect freeze-dried products from contamination during the freeze-drying process is described. The container has at least one side that includes a hydrophobic, porous, germ-tight, water vapor-permeable membrane. Water vapor can escape the closed container through this porous membrane, while the membrane represents a barrier to contamination. Another technique used, such as that taught in U.S. Pat. No. 5,309,649 to Bergmann, involves freeze-drying material in a container that has a porous hydrophobic wall. Neither of these patents, however, addresses the concern about re-hydrating the contents of the container once the doors of the drier are opened. It is not obvious how products freeze-dried in such a container could be kept dry and finally packaged in a vapor-tight container without first exposing the dried product to humidity. Thus, a need exists for a container for freeze-dried products that maintains a well-defined level of protection throughout the entire drying process, as well as providing means for forming a vapor-tight seal on the container before the dryer doors are open.